The invention relates to the technical domain of devices for air processing, and more particularly to devices for submitting a fluid charged with aerosol particles (specific dusts or molecules, etc.) to the action of an electrostatic field with high variations of amplitude and orientation, with a view to carrying out a specific electromagnetic action on these particles. This electromagnetic action on the particles can have the aim of mechanical, physical or chemical consequences etc.
The device for generating the electrostatic field according to the invention is of the type constituted of a porous electrostatic module in dielectric material, arranged in sandwich between two porous conducting electrodes, put under different electric voltages, to induce a distribution of charges and an electric field inside the electrostatic module. The charged fluid is pushed through the porous sandwich, in particular using a ventilator. Such devices for electrostatic action on aerosol particles are at present used mainly for electrostatic filtering of particles transported by an air flux. But they use a non-organised electrostatic module in a three-dimensional network and the actual geometry of the cells is random. They implement an electrostatic field which is weakly amplified locally and with small variations of amplitude and orientation.
The electrostatic module of a generator according to the invention is constituted of a meshed network, whose meshes have an organised three-dimensional periodicity and whose mesh cells have a recessed structure, convex externally and concave internally and compact, with a particular repetitive geometry for achieving the required amplification effect and electrostatic field structure.
The device for amplified electrostatic action according to the invention has much wider applications than those of the domain of filtration. Nonetheless, the closest prior art is essentially constituted of electrostatic filters for dusts. Consequently, the analysis of prior art given below is restricted to this technical field and more generally to the different techniques for filtering dusts.
For medical, sanitary and air purity reasons, it is advantageous to filter the small particles in the air and in particular industrial dusts, pollens, bacteria, viruses, fungi, algae and other fine dusts. Apart from systems operating using gravitation (settling of particles due to their weight) and cyclone systems operating through the effect of centrifugal forces (both of them used together in certain specific industrial applications, resulting in systems with large overall dimensions), the two most common methods for ensuring decontamination of dust particles in the air comprise:xe2x80x94one to block the air flow with the aid of a medium (mechanical filter);xe2x80x94and the other to deflect and capture the dusts using an electrostatic method (electro-filter).
According to the first method with a mechanical filter, the air passes through a porous filtering medium. A distinction can be made between: mechanical filters with surface filtration; and mechanical filters with in-depth filtration.
In the case of surface-effect mechanical filters, the medium is generally constituted of a sheet of woven metallic threads, a meshed material, a paper filter, a membrane etc. The medium has surface pores or holes. The size of the pores in the medium is chosen such that the air passes through whereas, because of their size, the xe2x80x9cbig particlesxe2x80x9d are captured and held on the fibres or material of the medium. Although, in practice, mechanical filters rarely have holes smaller than the size of the particles they have to capture, they act as if they had. The particles are captured progressively on the edges of the holes and tend to create a porous wall bridging the hole by agglutination. The collected particles cling together on the surface of the filtering medium and progressively constitute a xe2x80x9ccakexe2x80x9d of increasing thickness. Thus, with use, the xe2x80x9ccakexe2x80x9d of collected particles becomes the filter and the initial medium becomes the filter support.
In the case of mechanical filters with in-depth filtration, the particles do not only form a coherent surface cake, but the particles are caked to a certain depth in the vicinity of the surface of the medium. An example of such a filter is constituted by cigarette filters. The medium has a certain thickness and its fibres are not woven to form a surface, but are collected together in thickness according to more or less random directions. When the fluid meets a fibre placed approximately transversal, the flux is deflected around the fibre, but part of the particles, denser than the gas, have a tendency to be deflected less. They hit the fibre instead of following the gas. The separation of dusts is carried out by inertial impact on the fibres. The captured particles are then partly kept in place according to Van der Waals electrostatic forces.
In applications requiring high filtration efficiency, such as sterile rooms, a variant of the mechanical filter is used, applying folded filtering medium cartridges, called high efficiency filters (HEPA) or very high efficiency filters (ULPA).
The second method, with an electrostatic filter or electro-filter, is based on the fact that particles with a certain charge are attracted by a collector electrode of the opposite charge. This method has been used widely in industry since its invention by F. G. Cottrell in 1910. Preliminary means are used to give an electrostatic charge to the particles and, using an electrostatic field, these charged particles can be precipitated on a collecting wall or a collecting medium maintained at a voltage of the opposite sign. There exist two principal classes of electrostatic filter structures:xe2x80x94those with one stage,xe2x80x94and those with two stages. There also exist two variants of electrostatic precipitation means:xe2x80x94those with electrodes under voltage generated externally by electrical supply,xe2x80x94and those with electrostatic self-charging, charged by air friction.
Single stage electro-filters, or filters charged electrostatically, are produced according to the self-charging variant, according to which their filtering medium accumulates an electrostatic charge resulting from the passage of the air flux through the medium constituting them. In general they also use the fact that the particles are previously charged electrostatically from air friction. They are of low cost and of low efficiency.
Two stage electro-filters, also called electrostatic precipitators, are of higher complexity, cost and efficiency. They comprise a stage of electric charging of the particles by corona effect plus a precipitation stage. In the electric charging stage, the air passes through an ionisation zone constituted of one of several wires set at high voltage to generate an intense electrostatic field, within which the particles are electrically charged by ionisation. Then the air flux comprising the charged particles passes through a second collection stage. There are two types of two-stage electrostatic filters, depending on the structure of the collector stage for charged particles (xe2x80x94with plates,xe2x80x94or with a filtering medium).
According to the first type of electrostatic filter, with parallel plates but with electric field transversal to the flux (used in particular for treating industrial gaseous effluents), the collector is formed of an alternate parallel plurality of plates under high voltage and earthed plates, oriented parallel to the air flux. The charged particles are deflected perpendicularly to the flow path because of the transversal electric field, perpendicular to the plates and thus to the flow. As a result, the particles are precipitated onto the plates.
According to a second type of electrostatic filter, with filtering medium, the collector stage is constituted of one or several grids or porous electrodes (generally put under alternating voltages) and separated by plates of a filtering porous collector medium. The grid and the plates are located perpendicular to the air flow. The field is closely longitudinal or slightly inclined relative to the path of the fluid. This type of electro-filter is used principally for domestic purposes, especially in the field of air conditioning and central heating.
Among the different types of known filters and more generally among the electrostatic systems of prior art, the electrostatic filters with transversal medium, and as an auxiliary the in-depth filtration mechanical filters, are structurally the closest to the electrostatic device according to the invention.
A first major disadvantage of mechanical or electrostatic filters with a filtering medium is that the smallest particles and in particular the microbial particles (bacteria and viruses) pass through the pores, such that the efficiency of the mechanical filters diminishes considerably in function of the size of the particles treated.
Thus, in order to filter very small size particles, in particular bacteria and viruses, the size of the pores has to be reduced in the same proportion or the fibres have to be made denser. There is consequently a significant loss of pressure load during passage through the filtering medium, which brings about a considerably higher energy consumption.
This means that a second disadvantage of mechanical or electrostatic filters with a filtering medium is that they produce a loss of charge and high energy consumption.
A third disadvantage of mechanical or electrostatic filters is the fact that the filtering efficiency of physical methods implementing the filtering media (whether they operate by impact, diffusion or electrostatic effect) is poor. This means that the porosity of the filtering media (pore size) must be of the same order of size or, in all cases, must be sufficiently low relative to the size of the particles to be filtered. Consequently, the filtering media used have a low porosity ratio (empty pore volume for filtering material volume). This is disadvantageous and brings about:xe2x80x94low in-depth efficiency,xe2x80x94a low retention volume compared with the thickness of the medium, and thus compared with the resulting loss of charge,xe2x80x94and rapid clogging of the pores which quickly makes the filtering medium filter inefficient.
A fourth disadvantage of filtering medium filters, whether they be mechanical and of xe2x80x9csurface effectxe2x80x9d or xe2x80x9cin-depthxe2x80x9d effect or electrostatic, is that they retain the large particles more on the surface than in depth. They act essentially on the surface or over a small thickness near the input face. They quickly formxe2x80x94either a surface xe2x80x9ccakexe2x80x9dxe2x80x94or a layer of big dusts, clinging against the fibres, of density decreasing with the depth of the medium which, in the two cases, progressively blocks the porosity of the medium, reduces the cross-section of the pores considerably, increases the loss of charge and reduces the flow rate and the efficiency of the filter.
A fifth disadvantage, resulting from the preceding disadvantages, is that the filtering media need complete cleaning of the medium or frequent changing of this medium (HEPA or ULPA filter cartridges . . . ). This is both expensive and very difficult to implement reliably. This is the case, in particular, for food industry factories, or hospitals, where a large number of filters must be maintained or have their medium changed regularly because of progressive clogging.
A sixth disadvantage of filtering medium filters is their low reliability. In fact, the efficiency and blocking of filtering media are very sensitive to the ambient air, in particular its humidity and the particle concentration. These parameters are often random and in practice very difficult to take into account in evaluating the regularity of maintenance required.
A seventh disadvantage, related to electrostatic filters, whether they be of the plate or filtering-medium type, is linked to the deposit of big particles on the electrodes or on the active parts of the filtering medium, with consequent progressive lowering of the efficiency of the filter by dielectric effect (as described below). The result of particle deposit is the progressive formation of a dielectric surface film of dust. The adhesive forces of electrical cohesion must be sufficient to prevent these particles being drawn into the air flux. One of the properties of the dust layer, which is extremely important for the operation of an electro-filter, is the electrical resistance of this layer. Because of the very wide spectrum of the filtered particles, the electrical resistance can vary from 10expxe2x88x923 to 10exp14 ohm.cm. When the resistance is very low (less than 10exp4 ohm.cm), there is very rapid movement of charges between the dusts deposited on the conducting plate. Insufficient electrostatic charge remains within the particles collected, to hold them. This results in frequent re-collection of the particles and the efficiency of the electro-filter suffers. On the other hand, if the efficiency of the dust-film is too high and greater than 10exp10 ohm.sec, the efficiency of the electro-filter is reduced considerably. In fact, a large fraction of the variation in electric potential takes place across the high resistance dust film and not in the air. The particles are attracted less because of this lowering in voltage. Furthermore the electric resistance of the layer varies with time. Consequently, an eighth disadvantage of electrostatic filters with plates or a filtering medium is that their electrostatic efficiency diminishes with time.
A ninth disadvantage of electrostatic air filters with filtering medium, according to prior art, is that they do not enable spatial selectivity of capture of particles according to their size, meaning that particles of any size are captured almost uniformly in each zone of the medium. The big particles (and the small ones) are deposited uniformly over the capturing surface of the filtering medium. Progressively, the small particles and also the big ones are (uniformly) captured less and less. In other terms the filtering of the small particles is hindered by the fact that the medium is encumbered by the big particles. When the air comprises a large spectrum of particle sizes, the action on the small particles is rendered inefficient progressively because of blocking by the big particles.
It is well known in prior art how to place porous filtering media sandwiched between two porous electrodes set at different voltages with the aim of creating an electric field inside the filtering medium, and how to make a flux of air loaded with particles pass perpendicularly through the sandwich. This is the classic structure of the collection stage of an electrostatic air filter with transversal medium. The U.S. Pat. No. 3,999,964 describes an electrostatic air filter with transversal medium of this type, comprising a medium constituted of a porous material sandwiched between two V-shaped and perforated surface grids. One of the grids is set at a voltage of 6000 V whilst the other is earthed. The air is forced through the sandwich constituted by the two grids and the medium. U.S. Pat. No. 5,108,470 also describes a system of this type. Such a system is also described in the U.S. Pat. No. 5,330,559. An application of this technique is also described in the European patent WO 93/23171 in the name of the inventors.
U.S. Pat. No. 5,368,635 and U.S. Pat. No. 5,540,761 describe such a system in which, furthermore, the particularity is to slow down the gas at the level of the medium, in such a way as to allow a greater transversal movement of the particles, in order to raise capture efficiency and to make it possible to increase the size of the pores by thus limiting the speed of clogging of the pores by the dusts. In a variant it is recommended to use a medium constituted by association of conducting and/or insulating fibres placed at random in such a way as to provide holes in which xe2x80x9cintense fieldsxe2x80x9d develop. Different types of materials are suggested (paper, glass fibre, natural fibres, . . . ) whose structure is essentially random, meaning without any defined geometric organisation.
It is also known in prior art how to constitute an electro-filter with transversal medium by passing a flux of air loaded with particles through a porous collecting medium, self-charged electrostatically (one stage electro-filter), where the collecting medium comprises an assembly of channels constituted by the random structure of the medium in cellular foam material. In particular, it is known how to use a synthetic foam with open cells as filtering medium. Thus, to constitute an electrostatic filter, the U.S. Pat. No. 4,115,082 proposes placing two adjacent sheets, made of xe2x80x9csynthetic foam with open cellsxe2x80x9d to cover the assembly with two films of synthetic resin fibres capable of maintaining a negative charge, and to place the whole assembly between two series of acrylic plastic rods capable of developing a positive electrostatic charge. U.S. Pat. No. 5,336,299 describes a self-charged electro-filter of the same type, whose filtering medium is constituted of a xe2x80x9ccentral film in Plexiglas honeycomb weavexe2x80x9d.
Prior art is not concerned either by the particular overall geometry of the mounting of the cells of the porous material used (periodicity) or the internal organisation of each of these cells (geometry).
Thus, the U.S. Pat. No. 4,115,082 recommends using xe2x80x9ca foam with open cells in polyurethanexe2x80x9d without referring to the importance of the particular geometry or the organisation of the cells. The same applies to the U.S. Pat. No. 5,336,299.
It is known how to pass the fluid through parallel channels with wide elongated rectangular cross-section. An electro-filter of this type is described in the U.S. Pat. No. 4,007,024. The passage channels are provided between a plurality of elongated parallel collector plates separated from each other and including an ionising wire in the centre. An equivalent system is described in the U.S. Pat. No. 5,198,003. An equivalent system is also described in U.S. Pat. No. 5,484,473.
It is known how to make the fluid pass close to elongated parallel plates which can be described as xe2x80x9cwingsxe2x80x9d inclined relative to the path of the fluid and provided with tapered trailing edges. U.S. Pat. No. 4,007,024 describes such parallel plates provided with tapered trailing edges, arrow shaped, in such a way as to deflect and slow down the fluid. These xe2x80x9cwings are not set in a three-dimensional manner but two-dimensionally.
It is known in prior art how to use a xe2x80x9choneycombxe2x80x9d material as collector medium constituted of parallel channels which can be called parallel open elongated cells. U.S. Pat. No. 4,205,969 proposes placing a collector medium constituted of one or several plates of a xe2x80x9choneycombxe2x80x9d dielectric material between two metallic electrodes, also in xe2x80x9choneycombxe2x80x9d. The xe2x80x9choneycombxe2x80x9d material is constituted of elongated parallel channels opening into each other, along the axis of the fluid or slightly inclined. Such a particularity is also described in the U.S. Pat. No. 3,988,131. These cells have a two-dimensional and not a three-dimensional repetitiveness.
It is known how to make a fluid pass between wide and long parallel plates, submitted to differences in voltage. An electro-filter with two stages and with a plate of this type is described in the U.S. Pat. No. 4,259,093. In this case the repetitiveness is one-dimensional.
It is known how to give the filtering medium the form of a layer with a fine open structure, in woven wires. The U.S. Pat. No. 5,037,455 proposes such a structure made of woven polypropylene. This medium does not providexe2x80x94cells.
It is known in prior art how to use, as collector surface, an assembly of very elongated parallel channels in the form of tubes, placed between the electrodes; the channels and the electrodes being parallel to the movement of the fluid. The U.S. Pat. No. 4,234,324 proposes such a structure. U.S. Pat. No. 5,198,003 also describes a similar structure as does the U.S. Pat. No. 4,284,420.
It is known how to place a fine filtering medium of the HEPA type between live electrodes to increase filtering efficiency. This is described in the U.S. Pat. No. 4,357,150 and the U.S. Pat. No. 4,509,420.
It is known in prior art how to place a micro-porous powder material arranged according to random geometry between two electrodes at different electric potentials and to make a fluid loaded with particles pass through. Such a device is described in the U.S. Pat. No. 4,224,710 in which the micro-porous powder material is constituted in particular of charcoal.
It is known in prior art how to submit a fluid to a tortuous passage, through a porous dielectric material placed between two live electrodes and to operate locally inclined fields within this dielectric. Such a device is described in the U.S. Pat. No. 4,759,778.
It is known how to provide the electrodes of an electro-filter with filtering medium with points but with the aim of encouraging ionisation of the air particles. This is described, in particular, in the U.S. Pat. No. 5,573,577. But prior art does not take into consideration the particular combination between electrodes with points co-operating with a special geometry of the filtering medium, also with points, to increase the field effect within the medium.
The invention relates to a volume generator of chaotic electrostatic field, ensuring local amplification of the electrostatic field, to submit a fluid loaded with aerosol particles to the action of an electrostatic field with high local variation in amplitude and orientation.
An electrostatic field generator according to the invention is of the type known in prior art comprising:
an induced electrostatic module, constituted of a porous material,
two electrostatic inducing electrodes placed facing each other, separated from each other, on either side of the electrostatic module,
a source of electric current,
at least two conductors connecting the terminals of the source to the electrodes, and
a means for putting the fluid at overpressure to ensure its flow through the electrodes and the electrostatic module.
In certain applications, one can add an ioniser upstream from the generator to pre-charge the particles electrically and to increase their interaction with the electrostatic field generated.
The invention relates principally to an improvement of the constitutive structure of the electrostatic module of the electrostatic field generator. In its general embodiment, the invention recommends using an electrostatic module constituted of a network of three-dimensional meshes showing (at least locally) a periodicity (or pseudo-periodicity) in three dimensions (in at least three directions). According to the invention, each mesh of the three-dimensional network is constituted of an externally convex elementary cell, recessed at the centre and therefore concave internally, to include a compact elementary empty cellular volume. This means that the transversal dimensions of the cellular volume are of the same order of size in the three directions. According to the invention, the elementary empty cellular volume of the majority of the cells located at the centre of the electrostatic module open out facing elementary empty volumes of neighbouring cells by at least four recesses through their elementary surface.
According to a preferred embodiment of the invention, the electrostatic module is constituted by assembling a plurality of fins with longilineal portions, constituted of a dielectric or semiconducting material. The fins have a fine, not very wide, transversal cross-section, of a much lower thickness than their longitudinal dimension, and comprise at least one lateral trailing edge, elongated and tapered. The fins are physically and electrically connected together by each of their extremities to constitute a dielectric three-dimensional network. In the three directions, the network can have either a strictly repetitive mesh or a quasi-repetitive mesh (quasi-network). The fins are associated and regrouped geometrically in order to constitute a multiplicity of elementary cells (network meshes). The majority of the interior fins of the electrostatic module are common to several elementary cells.
According to this preferred variant of the invention, the majority of the associated fins, belonging to a same internal cell of the electrostatic module, surround and juxtapose tangentially, along at least one of their lateral longitudinal faces, an elementary surface including an elementary empty cellular volume.
An important variant of the invention is that the internal elementary volumes of the cells have a convex and compact structure. By compact, it is understood that the transversal dimensions of elementary cellular volume are of the same order of size in the three geometric directions. By convex, it is understood that in the geometric mathematical meaning of the term the elementary volumes have an overall form close to a strict or slightly deformed ball, to an ellipsoid or to a regular and non-elongated parallelepiped volume, such as for examplexe2x80x94a tubexe2x80x94or a non-structured volume such as that constituted by the interstices embodied between a multitude of fibres regrouped at random. Finally, the communicating recesses between neighbouring cells are surrounded by the lateral edge of fins belonging to its cell and common to those xe2x80x2of the neighbouring cells.
An electrostatic field generator according to the invention comprises, within its electrostatic module, a three-dimensional plurality of zones of electrostatic induction, distributed over three-dimensional, periodic or pseudo-periodic networks. In the preferred embodiment according to the invention described above, the induction zones are located closely around the cellular volumes and in the vicinity of the trailing edges of the fins, at the interface between the cells.
These electrostatic induction zones crossed by the particles have high local variations of electrostatic field relative to the average intensity evaluated over the totality of the electrostatic module, and/or high variations of orientation of the electrostatic field relative to the average electric field orientation, evaluated over the totality of the electrostatic module.